The term crossflow filtration is applied to filtrative processes in which the fluid flow through a semipermeable barrier is accommpanied by a transverse flow over the plane of the barrier. The purpose of the transverse flow is to create hydrodynamic conditions which maintain or minimize the concentration of certain components within the fluid boundary layer adjacent to the semipermeable barrier. This effect maintains a desired level of fluid permeation through the semipermeable barrier and is essentially independent of the nature of the barrier. For microfiltration, the fluid to be treated contains microparticulate matter, and the barrier or membrane may contain holes or pores as small as 250 A. Membranes with smaller openings are used for the process of ultrafiltration which removes dissolved macromolecular species. When the openings are smaller than 100 A the membrane will show some exclusion of small molecules and ions, and the process is given the name reverse osmosis or hyperfiltration. The nature of these processes is generally described in S. T. Hwang and K. Kammermeyer, "Membranes in Separations", Vol. VII in the series "Techniques in Chemistry", Ed. A. Weissberger, J. Wiley & Sons, (1975).
Included among these cross-flow filtration configurations or geometries are the large tubular membrane modules, the plate and frame modules and the spiral wound modules. A brief identification of each of these membrane configurations along with associated advantages and disadvantages in performance and operation will next be given.
The cylindrical tubular membrane is generally cast onto the inside of a porous backing tube or inserted loose into a porous tube and sealed at both ends. The liquid carrier is fed through the central opening of the membrane and permeates out radially through the walls of the membrane. The unfiltered liquid carrier is then continuously recirculated through a closed path until the desired amount of filtration is achieved. The advantages of a cylindrical tubular membrane is that it is easy to clean physically and chemically, does not easily block and is simple to remove and replace. Its disadvantages lie in the fact that it has a relatively small membrane surface-to-volume ratio, and has a relatively large "hold-up volume" of fluid within it.
The plate and frame configuration tangential flow module comprises a set of spaced parallel plates of membrane material with spacers interspersed between the plates. The liquid carrier is passed across the surfaces of the membranes at high velocity. The liquid that permeates through the membranes is drawn off through an interspace between coupled pairs of membranes. The advantages of the plate and frame filtration module include its relatively high membrane surface-to-volume ratio, low process fluid pumping costs, and ease of isolation of small areas in the event of leakage. Its disadvantages lie in its generally uneven flow patterns which facilitates stagnation in some areas and difficulty in cleaning and blocking due to the narrow channel through which the liquid carrier has to pass.
The spiral wound filtration module is formed by interleaving a porous sheet between two rectangular sheets of membrane material and sealing three sides of the resultant sandwich-like arrangement. The unsealed side is placed in fluid communication with a cylindrical tubular member that forms a header for the removal of permeate or filtrate. The sandwich-like structure is rolled in spiral fashion around the cylindrical tube. The liquid carrier to be filtered is pumped through the spiral-wound filtration module from one end to the other along the longitudinal axis of the module. As the liquid carrier passes over the membrane surface liquid permeates through the surface to the porous sheet which functions as a permeate collection material. The permeate flows in circular, spiral-like fashion until it reaches the inner tubular member where it is collected and channeled out of the filter. The advantages of the spiral-wound filtration module include its high membrane surface-to-volume ratio. However, that is offset by two distinct disadvantages. First, the spiral-wound module design requires that the permeate flow circularly around the spiral until it reaches the cylindrical tubular member where it can exit from the filter, and this relatively lengthy circular flow path can involve a significant pressure drop. Secondly, even one small leak in the spiral-wound filtration module generally requires the isolation of a significant portion of the total membrane area to identify and correct the leak.
Against this background of the prior art, an objective of the present invention is to provide a cross-flow filtration module design that has a relatively high membrane surface-to-membrane volume ratio, has relatively low susceptibility to blockage, does not require a significant pressure drop for the discharge of permeate or filtrate, and has ease of isolation of small areas in the event of leakage.